(1) The path length for water transport outside the xylem, and therefore the values of Kox, Kleaf, and Kplant, are defined by where evaporation occurs; for example, if evaporation occurs close to the epidermis, then K will be smaller, all else being equal, because water will have to travel farther before evaporating
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ψ at any given point in the leaf is influenced by both liquid and vapor phase transport, so the driving force for water transport and Kleaf are not causally related to the location of the evaporating sites; whether the term hydraulic conductance should be restricted to include only liquid phase pathways is a subjective matter; we argue that it is simpler to refine our interpretation of hydraulic to include vapor transport pathways, because they influence ψ values just as liquid pathways do and because operational measurements of hydraulic conductances often include contributions from vapor transport
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(2) If the pressure chamber estimate of leaf ψ (ψeq) does not coincide with the ψ at the sites of evaporation, then ψeq will underestimate or overestimate the true driving force for water transport and, thus, overestimate or underestimate, respectively, the true values of Kox, Kleaf, and Kplant
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(3) The drawdown in ψ from the xylem to any given tissue will increase if the evaporation rate from that tissue increases, even if the overall transpiration rate does not change, because increased evaporation from a tissue implies increased flow through the hydraulic resistances proximal to that tissue
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The ψ drawdowns to different tissues are largely unaffected by shifts in the location of the evaporating sites for a given total evaporation rate from the whole leaf (see Fig. 13), because such shifts are driven primarily by changes in the magnitude of anisothermal vapor transport (AVT), which is not affected directly by ψ
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(4) Solutes dissolved in liquid water will tend to accumulate near the sites of evaporation (but will disperse by diffusion), because such solutes typically have negligible vapor pressures and, thus, remain in solution when a portion of their solvent (i.e. water) evaporates
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No modification
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(5) The effective diffusion length for water enriched in heavier isotopologs will be greater if the evaporating sites are farther from the xylem (i.e. closer to the stomatal pores), because enrichment occurs primarily at the evaporating sites
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(6) If evaporation occurs closer to the stomatal pores, then the pathways for (liquid) water transport and for inward CO2 diffusion will overlap to a greater extent, possibly helping to explain the observed correlations between measured Kleaf and mesophyll conductance to CO2 (gm)
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A correlation between Kleaf and gm could arise due to any overlap between the pathways for CO2 diffusion and water transport, whether liquid, vapor, or both; however, Kleaf-gm correlations would not imply the mutual involvement of aquaporins if CO2 pathways overlapped primarily with vapor phase water pathways
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(7) The opportunity for diffusive interference between water vapor efflux and CO2 influx will be greater if the evaporating sites are farther from the stomatal pores, because this will increase the overlap between the diffusion pathways for water vapor and CO2
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This is correct if shifts in the location of the evaporating sites are accompanied by differences in vapor flux (flow per unit of area in the intercellular airspaces), as may occur when mesophyll evaporation is favored by increased photosynthetic photon flux density (PPFD), but incorrect when differences in the location of the evaporating sites result solely from differences in the airspace fraction (as may apply to comparisons between species)
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(8) The gas-exchange estimate of gs is lower than the true conductance through stomatal pores to the degree that the evaporating sites are located farther from the stomatal pores, because gs measures diffusion from the sites of evaporation to the outside of the pores
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The gs value inferred by gas exchange describes diffusive pathways that begin at some location within the intercellular airspaces (specifically, where relative humidity is 100% as calculated based on the measured leaf T) and end just outside the stomatal pores; the origin of those pathways within the leaf can vary independently from, and even in opposite directions to, the location of the evaporating sites (see Figs. 8 and 12), so measured gs and ci are not related directly to the sites of evaporation; despite these shifts, the intercellular airspaces remain close to saturation, and inferred gs only slightly underestimates the conductance of the (shorter) diffusive pathways that extend only across the stomatal pores themselves |
(9) The gas-exchange estimate of ci is larger than the true value of ci prevailing in the mesophyll if the evaporating sites are closer to the stomatal pores, because ci is estimated using gs
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| (10) The intercellular airspaces in tissues closer to the transpiring epidermis will be farther below 100% relative humidity (calculated at the T of the lower leaf surface) to the extent that evaporation occurs farther from the transpiring epidermis |
